Multi-spectrum retrieval of Venus IR surface emissivity maps from VIRTIS/VEX nightside measurements at Themis Regio
Multi-spectrum retrieval of Venus IR surface emissivity maps from VIRTIS/VEX nightside measurements at Themis Regio
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DOI：
10.1016/j.icarus.2015.10.014
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发表时间：
2016-02-01
期刊：
影响因子：
3.2
通讯作者：
Haus, Rainer
中科院分区：
文献类型：
--
作者：
Kappel, David;Arnold, Gabriele;Haus, Rainer
红外线的表面发射率图有助于探索金星的地质。金星快车（VEX）上的可见光和红外热成像光谱仪（VIRTIS-M-IR）的IR映射通道获得的在忒弥斯地区的夜侧辐射光谱被用来从三个可访问的光谱表面窗口在1.02，1.10，和1.18 μ m的发射率数据。通过应用全辐射传输模型来模拟测量的光谱。在忽略地质活动的情况下，利用多光谱反演算法确定地表目标的发射率图作为覆盖该目标的许多光谱分辨率图像的参数向量，由于其他参数的强烈干扰，很难获得绝对发射率值。目标的真实发射率平均值不能被检索，也不能严格地预设检索地图的发射率平均值。检索的地图可以显示可能是人为的纬度和地形的趋势。一旦在后处理步骤中去除了趋势，就可以观察到，围绕其平均值的所得空间发射率波动的幅度随着平均值的增加而增加。应用线性变换，该线性变换将去趋势图转换为表现出被称为参考发射率的定义的发射率平均值，这里为0.5，从而产生具有相应变换的波动的“重正化发射率图”。经验证，重正化发射率图在很大程度上独立于重正化前的发射率平均值、干扰大气、表面和仪器参数的修改以及检索管道和数据校准和预处理的选定细节。该方法有效地避免了由于模型参数不完善或未考虑而引起的发射率反演误差。如果已知目标的给定面元处的绝对发射率，则可以根据所述变换来计算整个目标的绝对发射率图，假设没有随纬度和地形的真实趋势。在此之前，重整化发射率被解释为相对于参考发射率的空间变化。它们代表了一个重要的一步，对检索的绝对emiratis.Renormalized发射率图的Themis Regio在三个表面窗口确定从64测量重复。检索误差估计的统计评估的地图来自各种不相交的选择光谱和使用不同的假设上的干扰参数。三个表面窗口的双标准差误差分别为3%、8%和4%，允许地质解释。与早期基于合成光谱的误差分析结果的比较表明，未考虑的干扰大气参数的时间变化是一个主要的误差源。对应于未风化花岗岩和玄武质岩石之间差异的1.02 μ m表面发射率（20%）的空间变化很容易检测到，但研究目标区域排除了此类变化。在1.02和1.18 μ m处均检测到高达8%的高程异常。在目前的灵敏度，没有发现异常在1.10 μ m，但超过确定的误差水平的异常可以排除在外。具有单标准差显著性，所有三张图都显示出有趣的空间发射率变化。(C)2015 Elsevier Inc. All rights reserved.
Surface emissivity maps in the infrared can contribute to explore Venus' geology. Nightside radiance spectra at Themis Regio acquired by the IR mapping channel of the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS-M-IR) aboard Venus EXpress (VEX) are used to derive emissivity data from the three accessible spectral surface windows at 1.02, 1.10, and 1.18 mu m. The measured spectra are simulated by applying a full radiative transfer model. Neglecting geologic activity, a multi-spectrum retrieval algorithm is utilized to determine the emissivity maps of the surface target as parameter vectors that are common to many spectrally resolved images that cover this target.Absolute emissivity values are difficult to obtain due to strong interferences from other parameters. The true emissivity mean of the target cannot be retrieved, nor can the emissivity mean of a retrieved map be strictly preset. The retrieved map can exhibit trends with latitude and topography that are probably artificial. Once the trends have been removed in a post-processing step, it can be observed that the magnitude of the resulting spatial emissivity fluctuations around their mean value increases with increasing mean value. A linear transformation is applied that converts the de-trended map to exhibit a defined emissivity mean value called reference emissivity, here 0.5, yielding the 'renormalized emissivity map' with accordingly transformed fluctuations. It is verified that renormalized emissivity maps are largely independent of the emissivity mean before renormalization, of modifications to interfering atmospheric, surface, and instrumental parameters, and of selected details of the retrieval pipeline and data calibration and preprocessing. Extremely large emissivity retrieval errors due to imperfect or unconsidered forward model parameters are effectively avoided. If the absolute emissivity at a given bin of the target were known, the absolute emissivity map of the entire target could be computed according to the mentioned transformation, assuming absent true trends with latitude and topography. Until then, the renormalized emissivities are interpreted as spatial variations relative to the reference emissivity. They represent an important step toward the retrieval of absolute emissivities.Renormalized emissivity maps of Themis Regio at the three surface windows are determined from 64 measurement repetitions. Retrieval errors are estimated by a statistical evaluation of maps derived from various disjoint selections of spectra and using different assumptions on the interfering parameters. Double standard deviation errors for the three surface windows amount to 3%, 8%, and 4%, respectively, allowing geologic interpretation. A comparison to results from an earlier error analysis based on synthetic spectra shows that unconsidered time variations of interfering atmospheric parameters are a major error source. Spatial variations of the 1.02 mu m surface emissivity of 20% that correspond to the difference between unweathered granitic and basaltic rocks would be easily detectable, but such variations are ruled out for the studied target area. Emissivity anomalies of up to 8% are detected at both 1.02 and 1.18 mu m. At present sensitivity, no anomalies are identified at 1.10 mu m, but anomalies exceeding the determined error level can be excluded. With single standard deviation significance, all three maps show interesting spatial emissivity variations. (C) 2015 Elsevier Inc. All rights reserved.